In an electric power steering device of a vehicle, an electric motor such as a blushless motor is provided for providing a steering mechanism with steering auxiliary force in accordance with a steering torque of a wheel. As a device for driving this motor, there is known a motor control device using a PWM (Pulse Width Modulation) control system.
Generally, the motor control device using the PWM control system is provided with an inverter circuit and a control part for controlling this inverter circuit. The inverter circuit is provided with a plurality of pairs (three pairs in the case of a three-phase motor) of an upper arm and a lower arm each having a switching element. Further, the control part calculates a target value of a current to be allowed to flow through the motor in accordance with a steering torque detected by a torque sensor, to generate a PWM signal having a predetermined duty based on a deviation between this target value and a value of the current actually flowing through the motor. The control part then controls an ON/OFF operation of each switching element in the inverter circuit by means of this PWM signal, and accordingly supplies a current from a power source to the motor via the inverter circuit, to drive the motor.
In the motor control device as described above, the current flowing through the motor is detected by means of a resistor for current detection, called a shunt resistor. Incidentally, during a period when all the switching elements on the upper stages in the inverter circuit are off or during a period when all the switching elements on the lower stages in the inverter circuit are off, the inverter circuit is in a regenerating state, and the current theoretically does not flow through the shunt resistor. That is, the circuit is in a state where the motor current is regarded as zero. However, due to such an influence that part of circuit outputs in the inverter circuit is not zero, the current may practically flow through the shunt resistor even during the above period. This current is referred to as a drift current. Although being a minute current, the drift current has an influence on steering auxiliary force since the current flows during a period when the detected current value should essentially be zero, causing a driver to have a feeling of strangeness in terms of steering feeling.
Thereat, it is necessary to compensate an offset based on the drift current, so as to prevent the steering auxiliary force from being influenced by the drift current. To this end, a value of the drift current, detected in the state of the motor current being regarded as zero, is referred to as a current offset value, and a detected current value in the case of the current flowing through the motor is compensated by means of the current offset value. Based on the compensated current value and a target current value, a voltage command value for feedback control is calculated, to control the current flowing through the motor based on this voltage command value. The below-mentioned Unexamined Japanese Patent Publication No. H08-47280, Unexamined Japanese Patent Publication No. 08-175405 and Unexamined Japanese Patent Publication No. 2001-95279 describe techniques concerning offset compensation based on a drift current.
Unexamined Japanese Patent Publication No. H08-47280 describes a control method for an AC servo motor, which detects an actual current of a motor to obtain offset data every time a voltage command becomes zero, updates a current offset value by means of a current offset value obtained from the offset data, and feeds back the updated current offset value to the current command, thereby to perform offset compensation corresponding to a drift.
Unexamined Japanese Patent Publication No. H08-175405 describes an electric power steering device provided with an offset compensation unit for taking as an offset compensation value a value of a motor current detection signal in a state where a motor current is regarded as zero, to correct a deviation signal between a target current signal and a motor current detection signal based on the offset compensation value.
Unexamined Japanese Patent Publication No. 2001-95279 describes a motor controlling method for monitoring a current flowing through a motor by means of a shunt resistor, measuring an output of the shunt resistor on such timing that a moment current flowing through this resistor practically becomes zero, to generate a signal for compensating an offset between an actual measurement output value and an ideal output value.
When the motor current is compensated by means of the current offset value, upon an abrupt change in current offset value in accordance with the drift current, the voltage command value also abruptly changes in association with the above change. Accordingly, the number of revolutions of the motor abruptly changes, thus destabilizing the steering feeling,